At present, ceramic powders used in most functional ceramic and functional coatings are multi-component composite powders synthesized by a solid-phase reaction method which is the most widely adopted in industrial productions. Different from gas reaction and liquid reaction, the solid-phase reaction includes two processes of chemical reaction and substance migration to a reaction zone, belonging to a heterogeneous phase reaction. Atoms and ions in a raw material can react by close contact and slow diffusion. Therefore, solid-phase particles participating in the reaction to contact to one another is a prerequisite of chemical action and substance transfer among reactants. The reaction mechanism of synthesizing multi-component powder by a solid-phase method starts at contact portions of particle surfaces of each raw material to generate new-phase products, and then the structural adjustment and crystal growth of product layers take place. When the product layers have a certain thickness, the diffusion of each raw material continues by the product layers, until the all forms a new-phase structure. In practical production, generally, powder raw materials of required components are ball-milled and mixed by a wet process and dried, and then are calcined to conduct the solid-phase reaction, to synthesize multi-component ceramic composite powder with a certain crystal structure.
Compared with the liquid-phase method or the gas-phase method, the solid-phase method has the advantages of no special requirements to process conditions, simple and convenient operation, wide availability of raw materials, low production cost, high efficiency and little pollution to environment. Especially in case of determining raw material components, the solid-phase method may relatively accurately control its component constitution, and thus has high universality and is suitable for industrial productions. Recently, the solid-phase method is still widely applied to the industrial production of general multi-component ceramic powder, and also often to laboratory studies.
However, this method has obvious disadvantages. During the process that slurry uniformly mixed-grinding by the wet process is dehydrated and dried, each component powder raw material tends to have the phenomenon of component sedimentation and non-uniform aggregation state due to different specific gravities and suspension properties. Generally, non-uniform components caused during this process is avoided by filter pressing, atomizing or freeze-drying, which certainly increases equipment investments and process costs; when synthesized powder is calcined, in case of natural stacking, there is poor contact among the multi-component powder, relatively distant diffusion distance between mass points, slow reaction speed, low efficiency and loss of volatile components. In order to facilitate the reaction, it is possible to refine granularity of raw powder as much as possible (for example, 1 μm or less), appropriately enlarge the contact surface of the reactants, and increase the mixing uniformity of the reactants or raise the calcination temperature. However, raising the calcination temperature not only increases the cost, but also causes hard aggregation in powder, relatively great powder granularity, and relatively wide particle size distribution; if calcination is conducted after briquetting, a local sintering cake is difficult to be broken due to non-uniform density of each position of a press cake although the reaction efficiency is increased. In summary, the traditional solid-phase reaction synthesizing method, when used to produce composite powder, has the disadvantages of low possibility of uniformly mixing each component, high synthesizing temperature, and great powder particle size. Besides, it is often impossible to get the required phase composition, and thus influence the quality of the synthesized powder.
Recently, the development of the technique of synthesizing ceramic powder by the liquid-phase method provides a better choice for preparing multi-component composite ultrafine powder. Since uniform mixing of each component at a level of molecule and atom can be realized in the liquid-phase, the synthesized powder has relatively good performance, thereby becoming a new technique widely applied in a laboratory room and in production. In the technique of synthesizing the multi-component composite ceramic powder by the liquid-phase method, a coprecipitation method and a sol-gel method are widely used. The coprecipitation method must adopt water-soluble raw materials, and it is easy to prepare the multi-component system powder. Recently, many practical applications have been found, but its coagulation and multiple filtering and cleaning are time-consuming and complicated. Moreover, for the multi-component composite system, due to different conditions of different metal ions in a solution generating sediments, it is scarcely possible to let plural ions of the constituent materials to subside at the same time. Meanwhile, a solubility product is different in sediments, and loss of partial components may occur in the water-washing process, which causes inaccurate components, and influences the performance of synthesized powder; the sol-gel method uses good diffusion of colloid ions, and may obtain nano ultrafine powder by adopting the appropriate dehydration and drying process. However, such process usually takes expensive metal alkoxide as a raw material, with high cost and long period. The gel process of sol is difficult to control. If the dehydration method is not appropriate, polycondensation and caking occur, which causes hard aggregation of particles, and thus the industrial production is limited.
For the problems existing in the traditional solid-phase reaction method and the prior liquid-phase method to synthesize the composite ceramic powder, a new powder synthesizing process is proposed. This method is a new powder preparing process generated by combining the traditional solid-phase reaction powder preparing process and the ceramic casting technique. The basic process of this technique is to mix the raw materials containing each component according to a certain ratio to prepare a aqueous slurry, add organic monomer acrylamide and a crosslinker of methylene bisacrylamide therein, make the organic monomer and the crosslinker conduct polymerization reaction under certain conditions to form a water-based polymer gel whose three-dimensional network skeleton fixes each raw material therein. The gel is burnt to remove the organic substance subjected to dehydration and drying at a certain temperature, and then to synthesize and obtain the required ceramic powder after calcination. In this process, the raw material slurry conducts rapid gelation reaction by controlling exterior conditions after uniformly mixed, and its drying process is conducted subjected to the gelation reaction after each raw material particle is fixed and cannot move relatively. Therefore, compared with the conventional solid-phase method, it can avoid the problem of non-uniform component caused by sedimentation significantly. Meanwhile, the raw material powder in the gel can keep a closely stacked state after dried. Close contact contributes to the solid-phase reaction in calcination, so the calcination synthesizing temperature is obviously lower than that of the conventional solid-phase method.
However, the above method needs to use acrylamide as a gel material, which is toxic and is harmful to human. In addition, together with a dispersing agent, pH value modifier, initiator and catalyst added when preparing the slurry, the total amount of the added organic matter will achieve more than 3% of the raw material powder weight. During the process of calcining the gel after dried, it needs to slowly raise its temperature or conduct temperature preservation at the temperature of 200° C. to 600° C., to thoroughly burn off the residual carbon of the organic matter, which prolongs the calcining time and does not contribute to the energy conservation.